The removal of hydrogen sulfide from gaseous streams produced in various industrial chemical processes, oil field production and petroleum processing has become increasingly important because of the limitations on the environmental liberation and/or burning of gaseous streams containing hydrogen sulfide. Hydrogen sulfide contributes to air pollution (when released to the atmosphere or burned), is hazardous to human health, and creates an odor nuisance in very low concentrations. Federal, state and local regulations exist restricting the amount of H.sub.2 S, as well as SO.sub.2, the combustion product of H.sub.2 S, that can be released to the atmosphere. Further, most natural gas sales contracts between the producers and end users, utilities, or pipeline companies require that the H.sub.2 S content be minimal, usually below 4 ppmv. Therefore, it is usually necessary to remove H.sub.2 S from these gases before end use, sale, or even emission to the atmosphere.
There are many processes known in the art for removing hydrogen sulfide from gaseous streams including caustic scrubbing, as exemplified by the disclosure in U.S. Pat. No. 2,747,962 to Heitz et al., scrubbing with aqueous solution of a water soluble nitrite such as sodium nitrite as disclosed in U.S. Pat. No. 4,515,759 to Burnes et al., and numerous other processes. The present invention relates to the use of an aqueous solution of a polyvalent metal chelate catalyst, such as a chelated iron redox catalyst, for the oxidation of hydrogen sulfide in a gaseous stream to produce elemental sulfur precipitate in the solution.
The use of a polyvalent metal chelate catalyst in aqueous solution to oxidize hydrogen sulfide in a gaseous stream to produce elemental sulfur in the solution is well known in the art and can be illustrated by the following representative disclosures which are incorporated herein by reference: U.S. Pat. No. 3,068,065 to Hartley et al., U.S. Pat. No. 3,097,925 to Pitts et al., U.S. Pat. No. 3,199,946 to Fujita et al., U.S. Pat. No. 3,676,356 to et al., U.S. Pat. No. 3,933,993 to Salemme, U.S. Pat. No. 4,009,251 to Meuly, U.S. Pat. No. 4,011,304 to Mancini et al., U.S. Pat. No. 4,036,942 to Sibeud et al., U.S. Pat. No. 4,189,462 to Thompson, U.S. Pat. No. 4,238,462 to Hardison, U.S. Pat. No. 4,356,155 to Blytas et al., U.S. Pat. Nos. 4,368,178, 4,382,918, 4,400,368, and 4,515,764 to Diaz, U.S. Pat. No. 4,374,104 to Primick, U.S. Pat. Nos. 4,390,516 and 4,414,194 to Blytas, U.S. Pat. No. 4,431,616 to Chou, U.S. Pat. No. 4,499,059 to Jones et al., U.S. Pat. No. 4,525,338 to Klee, U.S. Pat. No. 4,532,118 to Tajiri et al., U.S. Pat. No. 4,534,955 to Rosenbaum, U.S. Pat. No. 4,649,032 to Snavely et al., and "Shell Redox Desulfurization Process Stresses Versatility" by Fong et al., Oil and Gas Journal (OGJ Report), May 5, 1987, pp. 54-62.
As can be seen from the disclosures of the above patents and articles, the hydrogen sulfide removal processes using aqueous solutions of polyvalent metal chelate catalyst include various multistage processes for (a) oxidizing hydrogen sulfide to elemental sulfur in a gas-liquid contacting vessel, (b) separating the sulfur from the aqueous solution by settling, frothing, filtration or other means for removing the solid elemental sulfur, (c) regenerating the reduced polyvalent metal chelate catalyst to its oxidized form in a separate regeneration vessel and (d) recycling the regenerated metal chelate catalyst to the contacting vessel for further contact with the hydrogen sulfide containing gaseous stream. In order to effect the settling, frothing, flotation, filtration, hydrocloning or other separation of the sulfur from the solution, it is an objective of these prior art processes to agglomerate the sulfur particles or otherwise produce as large sulfur particles as possible to make the separation of the sulfur from the solution easier and more effective.
The amount of H.sub.2 S present in gas streams varies, as can the size of the gas stream, and thus the amount of H.sub.2 S that must be removed on a daily basis can vary from as high as thousands of tons per day to as little as a few pounds per day. Many commercial processes exist for H.sub.2 S removal, and the selection of the process is strongly dependent on the daily amount to be removed. For instance, for large daily amounts (greater than about 5 ton/day) the typical process configuration involves H.sub.2 S separation and recovery with an alkonolamine process followed by conversion to sulfur in the Claus process. However, for small scale processing (e.g., less than 500 lb./day H.sub.2 S) this scheme is expensive and impractical due to the complicated nature of the equipment. Thus, removal of small daily amounts of H.sub.2 S presents a special case for economical processing.
Several processes are commercially available for the removal of small amounts of H.sub.2 S. A key feature of these processes is that they are typically batch chemical type processes, with simple, low cost equipment. The "iron sponge" process uses iron oxide impregnated wood chips in a dry bed to remove H.sub.2 S from the gas that is passed through the bed by reacting the H.sub.2 S to form iron sulfide When the material is spent, it is removed from the bed and disposed of in an acceptable manner. The disadvantage with this process is that, although the chemical itself is relatively inexpensive, the loading and unloading procedures are cumbersome and labor intensive, plus the spent material is pyrophoric and often considered a hazardous waste.
Another commercial process uses an aqueous solution of sodium nitrite to react with H.sub.2 S when the sour gas is passed through a static fluid column in a single vessel. When the nitrite is all used up, the material is discharged and discarded as Class II waste, and a fresh charge is loaded into the vessel. A disadvantage of this process is that the sodium nitrite chemical solution is comparatively expensive.
The use of chelated iron a redox catalyst for hydrogen sulfide oxidation has been used in several commercial processes, which are based on the chemistry: ##EQU1## These processes involve complicated flow schemes, usually with separate absorbers, regenerators, settling tanks, pumps, and filters, centrifuges, etc., for recovery of the solid sulfur from the solution. Thus, they are capital intensive and complicated to operate. Therefore, these processes are generally not used for removal of small amounts of H.sub.2 S where simple processes like the "iron sponge" and sodium nitrite processes are favored.
It is an object of this invention to provide a simplified aqueous polyvalent metal chelate process for removing hydrogen sulfide from gaseous streams and to thereby provide a more economical process.
It is another object of this invention to provide an aqueous polyvalent metal chelate composition and method for removing small daily amounts of H.sub.2 S from gas streams on an economical basis.
It is a further object of this invention to provide an aqueous polyvalent metal chelate composition and a method for essentially complete removal of H.sub.2 S.
Another object of this invention is to enable the removal of H.sub.2 S in a simple process, using simple equipment that requires little operation attention and/or maintenance.
Yet another object of this invention is to provide catalytic method for H.sub.2 S conversion to sulfur in the presence of oxygen in a single gas/liquid contact vessel.
Another object of this invention is to disperse the precipitated sulfur to very small particles to prevent settling, foaming, frothing and fouling in the gas/liquid contact zone.